310 THE PRINCIPLES OF [Part II. 



excess over the other, have not been mastered by the sur- 

 vival of those varieties which were subjected to the least 

 waste of organized matter and force by the production of 

 superfluous individuals of either sex. Nevertheless we 

 may conclude that natural selection will always tend, 

 though sometimes inefficiently, to equalize the relative 

 numbers of the two sexes. 



Having said this much on the equalization of the 

 sexes, it may be well to add a few remarks on the regula- 

 tion through natural selection of the ordinary fertility of 

 species. Mr. Herbert Spencer has shown in an able dis- 

 cussion 74 that with all organisms a ratio exists between 

 what he calls individuation and genesis ; whence it follows 

 that beings which consume much matter or force in their 

 growth, complicated structure, or activity, or which pro- 

 duce ova and embryos of large size, or which expend 

 much energy in nurturing their young, cannot be so pro- 

 ductive as beings of an opposite nature. Mr. Spencer 

 further shows that minor differences in fertility will be 

 regulated through natural selection. Thus the fertility of 

 each species will tend to increase, from the more fertile 

 pairs producing a larger number of offspring, and these 

 from their mere number will have the best chance of sur- 

 viving, and will transmit their tendency to greater fer- 

 tility. The only check to a continued augmentation of 

 fertility in each organism seems to be either the expendi- 

 ture of more power and the greater risks run by the 

 parents that produce a more numerous progeny, or the 

 contingency of very numerous eggs and young being pro- 

 duced of smaller size, or less vigorous, or subsequently 

 not so well nurtured. To strike a balance in any case be- 

 tween the disadvantages which follow from the production 

 of a numerous progeny, and the advantages (such as the 



14 'Principles of Biology,' vol. ii. 1867, chaps. ii.-xL 



